Thermal benefits of vertical greening in a high-density city: Case study of Hong Kong

摘要

To improve outdoor thermal environment and reduce indoor energy use, passive techniques including facade greenery have been suggested. In high-density cities like Hong Kong, buildings' surface area is much greater than the roof and ground surface areas combined, offering a huge vertical surface platform for greening. However, scientific evidence to assert the thermal benefit from this greening option especially at neighborhood scale is still very few. Therefore, this study was designed to provide such evidence using results from validated ENVI-met model simulation. Thereafter, parametric study was conducted to investigate the quantity and location of facade greening required for potential air cooling and thermal comfort improvement of a neighborhood of varying densities. Model validation results revealed an acceptable modelling of facade surface temperature, air temperature, relative humidity and wall-emitted long-wave fluxes. From the parametric study, we found that 30-50% of facades in the high-density urban setting of Hong Kong must be greened to potentially cause -1 degrees C reduction in both daytime and nighttime air temperature while the same could help improve daytime pedestrian thermal comfort by at least one thermal class. We also established that higher greened facade ratio will be required to obtain similar thermal benefits in low and medium density urban settings. Also, realized benefits at pedestrians' height can be enhanced when the vertical greening facilities are placed along podium than tower heights. Lastly, practicable urban planning recommendations were presented for the attention of urban planners and landscape architects.